Through the Looking Glass (Star Trek: Deep Space Nine)

"Through the Looking Glass" is a two-part episode (episodes 19 and 20) from the third season of the science fiction television series Star Trek: Deep Space Nine. It explores the Mirror Universe, a parallel reality where the characters and factions are often morally inverted compared to their prime universe counterparts.

The episode begins with Jennifer Sisko, the deceased wife of Commander Benjamin Sisko, being pulled from the Mirror Universe into the prime universe by that reality's version of Benjamin Sisko. The Mirror Universe Sisko needs the prime universe Sisko's help to build a device that can save the rebels (primarily humans and Bajorans) from the oppressive Klingon-Cardassian Alliance, which is analogous to the Dominion in the prime universe. Jennifer serves as bait to lure Sisko into assisting.

Sisko agrees to travel to the Mirror Universe with Kira Nerys, who poses as her Mirror Universe counterpart, the Intendant. They encounter Mirror versions of several regular characters, including O'Brien, Bashir, and Quark. The episode focuses on the moral complexities and differences between the two realities, highlighting the brutal and opportunistic nature of the Mirror Universe's inhabitants. Kira, in particular, struggles with embodying the ruthless Intendant.

The device is eventually built, though not without significant risk and moral compromises. Sisko returns to his own universe with Jennifer, but she eventually chooses to return to the Mirror Universe to be with her son, Jake Sisko. "Through the Looking Glass" is a significant episode in the Deep Space Nine run, marking the beginning of a recurring arc focused on the Mirror Universe and providing deeper character development for Sisko, Kira, and the other actors who portray their mirror counterparts throughout the series. The themes of identity, choice, and the nature of good and evil are central to the episode.